使用Index提示 强制使用索引

使用Index提示 强制使用索引

Index Hints的格式如下:

虽然索引并不总会快于全表扫描,但是很多时候我们希望Oracle使用索引来执行某些SQL,这时候我们可以通过index hints来强制SQL使用index.

/*+ INDEX ( table [index [index]...] ) */

我们简单看一下这个提示的用法(范例为Oracle10g数据库):

SQL> create table t as select username,password from dba_users;

Table created.

SQL> create index i_t on t(username);

Index created.

SQL> set autotrace trace explain

SQL> select /*+ index(t i_t) */ * from t where username='EYGLE';

Execution Plan
----------------------------------------------------------
Plan hash value: 2928007915

------------------------------------------------------------------------------------
| Id  | Operation                   | Name | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT            |      |     1 |    34 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID| T    |     1 |    34 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN          | I_T  |     1 |       |     1   (0)| 00:00:01 |
------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

2 - access("USERNAME"='EYGLE')

Note
-----
- dynamic sampling used for this statement

这里的查询使用了索引.

需要注意的是使用CTAS方式创建数据表,新建表会继承原表的约束属性:

SQL> desc t
Name                                      Null?    Type
----------------------------------------- -------- ----------------------------
USERNAME                                  NOT NULL VARCHAR2(30)
PASSWORD                                           VARCHAR2(30)

如果不使用Hints,此处Oracle不会使用索引:

SQL> select * from t where username='EYGLE';

Execution Plan
----------------------------------------------------------
Plan hash value: 1601196873

--------------------------------------------------------------------------
| Id  | Operation         | Name | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |      |     1 |    34 |     2   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| T    |     1 |    34 |     2   (0)| 00:00:01 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

1 - filter("USERNAME"='EYGLE')

Note
-----
- dynamic sampling used for this statement

索引和全表扫描的选择和取舍并非简单,本文不作进一步探讨.